Lessons from Doom is a great article on some of the game design lessons learned from the classic game Doom.

So because the player moves so quickly in Doom, and because most enemy attacks are dodgeable, the player can avoid a significant amount of damage simply by moving. A skilled player can often deal with large numbers of enemies sustaining hardly a scratch. This creates a feeling that’s quite rare in modern FPS: that you are powerful because you are agile, not because you’re a tank. This frees up Doom’s encounters to feature huge numbers of enemies, to vary scenarios by mixing in different proportions of threats, and to have huge, sprawling, often non-linear spaces that the player can traverse easily. There’s nothing quite like it today.